Nuclear Materials and Energy 12 (2017) 478–482 Contents lists available at ScienceDirect Nuclear Materials and Energy journal homepage: www.elsevier.com/locate/nme Quartz micro-balance results of pulse-resolved erosion/deposition in the JET-ILW divertor ∗ G. Sergienko a, , H.G. Esser a, A. Kirschner a, A. Huber a, M. Freisinger a, S. Brezinsek a, A. Widdowson b, Ch. Ayres b, A. Weckmann c, K. Heinola d, JET contributors e,1 a Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung – Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), 52425 Jülich, Germany b CCFE, Culham Science Centre, OX14 3DB, Abingdon UK c Department of Fusion Plasma Physics, Royal institute of Technology (KTH), 100 44 Stockholm, Sweden d Department of Physics, University of Helsinki, P.O. Box 64, 00560 Helsinki, Finland e EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK a r t i c l e i n f o a b s t r a c t Article history: A set of quartz crystal microbalances (QMB) was used at JET with full carbon wall to monitor mass ero- Received 14 July 2016 sion/deposition rates in the remote areas of the divertor. After introduction of the ITER- like wall (ILW) Revised 8 February 2017 in JET with beryllium main wall and tungsten divertor, strong reduction of the material deposition and Accepted 9 March 2017 accompanied fuel retention was observed. Therefore the existing QMB electronics have been modified to Available online 22 March 2017 improve the accuracy of frequency measurements by a factor of ten down to 0.1 Hz which corresponds to − − − − 1.4 ng cm −2. The averaged deposition rates of 1.2–3 ng cm 2 s 1 and erosion rates of 5.6–8.1 ng cm 2 s 1 were observed in the inner divertor of JET -ILW with the inner strike point positions close to the bot- tom edge of vertical tile 3 and at the horizontal tile 4 respectively. The erosion with averaged rates of − − − ≈2.1 ng cm −2 s 1 and ≈120 ng cm 2 s 1 were observed in the outer divertor for the outer strike point po- sitions at tile 5 and tile 6 respectively. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license. ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) 1. Introduction change of the resonance frequency of the quartz crystal oscillator due to the change of the areal mass density of the layer deposited Plasma-wall interaction in fusion devices results in material on the quartz crystal surface. Unfortunately, the quartz crystal res- erosion of the first wall, its migration and deposition. According to onance frequency depends also on the crystal temperature and the present knowledge, tritium contained in the co-deposited lay- stress forces as well. To reduce the influence of the temperature ers, which are mainly formed in the remote areas, is responsible changes on the deposition rate measurements, water cooled AT-cut for the major part of the in-vessel tritium inventory. Control of the quartz crystals are usually used. Alternatively, an additional quartz tritium inventory is one of most important issues for the develop- crystal resonator protected against deposition is used for the tem- ment of fusion reactors with an acceptable level of environmen- perature compensation. Due to technical difficulties, water-cooled tal hazards. Material redistribution in tokamaks with carbon wall QMB systems are not possible to install in the divertor region of was investigated over decades and underlying physics has been the JET tokamak. Therefore the second method with two identical well understood. Understanding of these processes in tokamaks quartz crystal resonators, for the sensing and for the temperature with full metal first wall is important with respect to the ITER compensation, was applied [1] . project. The quartz crystal microbalance (QMB) method is widely used in industrial and laboratory applications for the measurement 2. Experimental set-up of the deposition/erosion in real time. This method is based on the A set of quartz crystal microbalances was used at JET with full carbon wall to monitor material erosion/deposition rates in the re- ∗ Corresponding author. E-mail address: [email protected] (G. Sergienko). mote areas of the divertor [1–7]. After installation of the ILW in 1 See the Appendix of F Romanelli et al., Proceedings of the 25th IAEA Fusion JET with beryllium main wall and tungsten divertor, a strong re- Energy Conference 2014, Saint Petersburg, Russia. duction (factor ∼10) of the material deposition and accompanied http://dx.doi.org/10.1016/j.nme.2017.03.007 2352-1791/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license. ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) G. Sergienko et al. / Nuclear Materials and Energy 12 (2017) 478–482 479 netic field. The QMB drive circuits provide stable 5 V power sup- ply and insulate the QMB signal lines from the JET vessel ground. The quartz crystals are not actively cooled and the temperature ex- cursion of the quartz crystal sensor during the exposure time can easily reach the Curie temperature (573 °C) after which permanent loss of their piezo-electric properties. Therefore, the shutter open- ing time should be properly chosen taking into account the quartz sensor’s distance to the strike point and additional heating power of the plasma. The original JET QMB diagnostics [1] had frequency measure- ment accuracy of about 1–3 Hz, which was sufficient for carbon layers erosion/deposition measurement. To improve the accuracy of frequency measurements by a factor of ten down to 0.1 Hz Fig. 1. Positions of QMBs in divertor. Divertor magnetic field configurations: red (which corresponds to the areal density change about 1.4 ng/cm 2 line for strike points at the corners, blue line for strike points on the vertical targets. or about 0.05 Be monolayer) the following modifications of the ex- (For interpretation of the references to colour in this figure legend, the reader is isting QMB electronics have been made: (i) all wired output of referred to the web version of this article.) QMB in-vacuo electronic boards were connected to the QMB com- mon input by 1 k load resistors, to reduce electromagnetic noise pickup and protect the QMB circuit against electrostatic overvolt- fuel retention was observed [8–10] . Four QMB sensors with gold age, (ii) pulse width discriminators were installed in both measure- electrodes equipped with shutters and compensation quartz crys- ment and reference QMB channels, which cut all noise pulses with tals (for reduction of temperature effects) were installed at the en- duration below 0.005 ms, (iii) introduction of an electronical cir- trance to the louvers behind the lower vertical targets of the in- cuit for overheating protection of the measurement quartz crys- ner (QMB1, QMB2, QMB3 at same poloidal but different toroidal tal, which closes the electromagnetic shutter when the QMB sen- locations) and outer (QMB5) divertor as shown in Fig. 1 . These sor frequency drops below 4 kHz, which corresponds to the crystal remote areas are accessible by neutral particles only. The QMB1, temperature of about 220 °C. The frequency measurements were QMB2, QMB3 and QMB5 have an identical construction. The QMB performed with help of the pulse counting techniques. The time in-vacuo electronics is based on single BiCMOS ASIC chip from SIN- interval for the pulse counting was increased from 0.3–1 s to 10 s TEF [11] , which contains the quartz crystal sensor oscillator, the to provide 0.1 Hz accuracy. The QMBs frequencies as function of compensation quartz crystal oscillator, the reference quartz crys- temperature were measured in laboratory by slow heating of the tal oscillator and two double balanced mixers. The latter are used QMB assembly in vacuum oven. The results of these measurements to generate the output signal of the difference between the fre- were used to improve the compensation of the temperature effects. quencies of the reference and both the deposition and compensa- The frequency change immediately after the plasma exposure is es- tion sensing crystals [12] . All quartz crystals have a resonance fre- timated by fitting within the time interval of 30 0–150 0 s using a quency of about 6 MHz with the reference crystals having higher combination of an exponential decay and a linear function. In ad- frequencies by about 10 kHz. This approach was used to reduce dition, the QMB frequencies were measured overnight and during QMB system signal frequencies because the in-vacuo QMB elec- weekends when the crystals cooled down to the same steady state tronic boards are connected to the ex-vessel QMB drive circuits by temperature of about 50 °C. The quartz crystal mass sensitivity co- − − means of 6 m long thermocouple cables, which are able to trans- efficient of 12.3 ng cm 2 Hz 1 was calculated using the Sauerbrey mit the signals only within 0–100 kHz frequency bandwidth. The equation [13] and was confirmed by weight change measurements frequency differences between sensor and compensation crystals of the quartz sensors (the same type as used in JET QMB diag- were about 10 0 0–150 0 Hz before the measurements. The chip with nostics) which were coated by different metal films (Cu, Cr and the reference quartz crystal is mounted on a printed ceramic board W) by means of magnetron sputtering [14] . Taking into account and placed in a copper box with two ceramic D-sub plugs. The the metal mesh transmittance, the mass sensitivity coefficient of − − first plug is used to connect the power supply with the two sig- 13.9 ng cm 2 Hz 1 was used for JET QMB data analysis.